In an oil-impregnated sintered bearing, the inside of a sintered body is impregnated with a lubricant in advance, the oil is caused to flow out by a pumping action by the rotation of a shaft and thermal expansion by friction heat, and a friction surface is lubricated. Such oil-impregnated sintered bearings can be used for a long period of time with no refueling and are thus being broadly employed as bearings for rotating shafts for automobiles, home appliances, acoustic devices, and the like (for example, refer to Japanese Examined Patent Application, Second Publication No. H8-19941).
In the case of supporting a rotating shaft using an oil-impregnated sintered bearing of the related art, for example, when a torque is transmitted to rotate the rotating shaft in a certain direction, a load in a shear direction is applied to the rotating shaft. At this time, when the shear load is extremely large or the stiffness of the rotating shaft is not high enough, the rotating shaft bends due to the shear load and rotates with the shaft line inclined inside the bearing, and there is a possibility that a state in which the surface of the rotating shaft does not properly come into contact with the friction surface inside the bearing (a motion of the rotating shaft hollowing the inner surface of the bearing) may be caused. When such a state is caused, the rotating shaft receives a strong resistance and is not capable of readily rotating, and the bearing does not sufficiently perform the function. In addition, when such a state is repeatedly caused, it can be also considered that the durability of the rotating shaft or the bearing degrades.
In order to overcome the disadvantage of the rotating shaft incapable of properly coming into contact with the friction surface inside the bearing in the case of receiving a load in a shear direction as described above, for example, an oil-impregnated sintered bearing including a straight hole portion having a constant diameter and an enlarged diameter portion having a diameter that increases outwards and forming a tapered shape in a bearing hole is known (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2004-308682).
A majority of oil-impregnated sintered bearings are formed of an iron (Fe)-copper (Cu)-based sintered metal. The Fe component is suitable for an operation state in which the rotation speed of the rotating shaft is slow and the load being applied to the rotating shaft is high. On the other hand, the Cu component is suitable for an operation state in which the rotation speed of the rotating shaft is fast and the load being applied to the rotating shaft is low.
In response to the recent increase in resource prices, particularly, a price increase of Cu, there has been a demand for additional cost reduction of products including Cu. Therefore, there has been another demand for oil-impregnated sintered bearings capable of dealing with the fast rotation speed of rotating shafts while decreasing the amount of Cu used. As oil-impregnated sintered bearing for which the amount of Cu used is decreased, Fe—Cu-based sintered metal bearings for which Cu-based flat raw material powder is used are being broadly used (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2006-299347).